Editing Parallax (section)

{{short description|Difference in the apparent position of an object viewed along two different lines of sight}}
{{about|the apparent displacement of an object viewed from different positions}}
{{More citations needed|date=April 2020}}
[[File: Parallax Example.png|thumb|upright=1.4|right|A simplified illustration of the parallax of an object against a distant background due to a perspective shift. When viewed from "Viewpoint A", the object appears to be in front of the blue square. When the viewpoint is changed to "Viewpoint B", the object ''appears'' to have moved in front of the red square.]]
[[File:Parallax.gif|thumb|right|This animation is an example of parallax. As the viewpoint moves side to side, the objects in the distance appear to move more slowly than the objects close to the camera. In this case, the white cube in front appears to move faster than the green cube in the middle of the far background.]]

'''Parallax''' is a displacement or difference in the [[apparent position]] of an object viewed along two different [[sightline|lines of sight]] and is measured by the angle or [[half-angle]] of inclination between those two lines.<ref>{{cite dictionary| title=Parallax | quote=Mutual inclination of two lines meeting in an angle | encyclopedia=Shorter Oxford English Dictionary | date=1968}}</ref><ref name=oed>{{cite dictionary| encyclopedia=Oxford English Dictionary | date=1989 | edition=Second | title=Parallax | quote=''Astron.'' Apparent displacement, or difference in the apparent position, of an object, caused by an actual change (or difference) of the position of the point of observation; spec. the angular amount of such displacement or difference of position, being the angle contained between the two straight lines drawn to the object from the two different points of view and constituting a measure of the distance of the object. | url=http://dictionary.oed.com/cgi/entry/50171114?single=1&query_type=word&queryword=parallax&first=1&max_to_show=10 }}</ref> Due to [[perspective (graphical)|foreshortening]], nearby objects show a larger parallax than farther objects, so parallax can be used to determine distances.

To measure large distances, such as the distance of a planet or a star from [[Earth]], astronomers use the principle of parallax. Here, the term ''[[Stellar parallax|parallax]]'' is the semi-angle of inclination between two sight-lines to the star, as observed when Earth is on opposite sides of the Sun in its orbit.{{efn|In the past [[#Diurnal parallax|diurnal parallax]] was also used to measure distances to celestial objects within the [[Solar System]]. This method has now been superseded by more accurate techniques.}} These distances form the lowest rung of what is called "the [[cosmic distance ladder]]", the first in a succession of methods by which astronomers determine the distances to celestial objects, serving as a basis for other distance measurements in astronomy forming the higher rungs of the ladder.

Because parallax is weak if the triangle formed with an object under observation and two observation points has an angle much greater than 90°, the use of parallax for distance measurements is usually restricted to objects that are directly "faced" by the baseline (the line between two observation points) of the formed triangles.

Parallax also affects optical instruments such as rifle scopes, [[binoculars]], [[microscope]]s, and [[twin-lens reflex camera]]s that view objects from slightly different angles. Many animals, along with humans, have two [[eye]]s with overlapping [[visual perception|visual fields]] that use parallax to gain [[depth perception]]; this process is known as [[stereopsis]]. In [[computer vision]] the effect is used for [[computer stereo vision]], and there is a device called a [[Coincidence rangefinder|parallax rangefinder]] that uses it to find the range, and in some variations also altitude to a target.

A simple everyday example of parallax can be seen in the dashboards of motor vehicles that use a needle-style mechanical [[speedometer]]. When viewed from directly in front, the speed may show exactly 60, but when viewed from the passenger seat, the needle may appear to show a slightly different speed due to the angle of viewing combined with the displacement of the needle from the plane of the numerical dial.